Sputtering is the deposition of a thin film of a desired material on a substrate. Sputtering is employed to manufacture semi-conductor devices such as integrated circuits. In a sputtering operation, the material to be deposited on the substrate is removed from a sputter target by bombarding the target with ions. In addition to removing material from the target for deposit on the substrate, ion bombardment transfers thermal energy to the target. It is therefore generally necessary to provide a means of cooling the target during the sputtering operation to prevent the target from overheating.
The target is traditionally mounted to a backing plate. Cooling liquid is circulated in heat transfer contact with the backing plate to remove heat transferred to the target by the bombarding ions. It is important that there be good thermal, as well as structural, bonding between the target and backing plate to ensure adequate heat transfer from target to backing plate to coolant and to ensure the structural integrity of the target/backing plate connection during sputtering.
One technique for bonding the target to the backing plate is known as diffusion bonding. In diffusion bonding the target/backing plate bond is produced by pressing the material surfaces into intimate contact while applying heat to induce metallurgical joining and diffusion to varying extent across the bond interface. Bonding aids, metal combinations which are more readily joined, are sometimes applied to one or both of the surfaces to be bonded. Such coatings may be applied by electroplating, electroless plating, sputtering, vapor deposition or other usable technique for depositing an adherent metallic film. It is also possible to incorporate a metallic foil between bonding members which foil has the ability to be more easily bonded to either of the material to be joined. The surfaces to be joined are prepared by chemical or other means to remove oxides or their chemical films which interfere with bonding.
The target/backing plate assembly may be diffusion bonded using techniques such as hot isostatic pressing ("HIPing") or uniaxial hot pressing ("UHPing"). In UHPing, the unbonded assembly is placed between a pair of plungers, platens or rams. These rams are contained within a control chamber which allows for the control of temperature, pressure and other atmospheric conditions. The controlled atmosphere is a vacuum, reducing gas or inert gas, or a combination thereof. The temperature in the uniaxial hot press control chamber is increased in order to heat the unbonded assembly. The assembly is heated to a temperature somewhat below the homologous melting point of the metal used for the backing plate. By elevating the temperature of the assembly to a temperature somewhat below the melting point of the backing plate material, the backing plate softens, and upon pressing, forms a tight interface with the treated bonding surface of the sputter target. As the assembly is heated, a compressing force is applied on the assembly by the rams in a uniaxial direction. The assembly is maintained in the control chamber under these temperature, pressure and atmospheric gas conditions for a period of time sufficient to form the bonded sputter target/backing plate assembly.
In HIPing, the treated sputter target and backing plate are oriented to form an assembly having an interface defined by the bonding surfaces, and this assembly is placed within a HIPing canister. Once the assembly is placed in the HIPing canister a vacuum is pulled on the canister and the canister is then placed within a HIPing chamber. The ambient atmosphere in the HIPing chamber is replaced with a true inert gas, such as argon or helium. In addition, the temperature and pressure in the HIPing chamber are increased as discussed above with respect to UHPing, in order to form a bonded sputter target/backing plate assembly. Again the assembly is heated to a temperature somewhat below the homologous melting point of the metal used for the backing plate, and the HIPing canister and assembly contained therein are compressed from all sides at elevated pressure. The assembly is maintained at the desired temperature, pressure and atmospheric conditions for a sufficient period to form the bonded target/backing plate assembly.
Representative times, temperatures and pressures for UHPing and HIPing are disclosed in U.S. Pat. No. 5,836,506, issued Nov. 17, 1998, entitled "Sputter Target/Backing Plate Assembly And Method Of Making Same", assigned to the assignee of the present invention and hereby incorporated by reference herein as fully set forth in its entirety.
Due to coefficient of thermal expansion induced shrinkage of the bonded target/backing plate during cooling subsequent to the diffusion bonding, any mismatch between the thermal expansion coefficients of the target and backing plate creates stresses in the assembly which can distort the assembly and/or crack the target.
One particular material from which targets are fabricated is tungsten-titanium. Target/backing plate assemblies wherein the target is fabricated from tungsten-titanium have typically employed titanium as the backing plate material to avoid a coefficient of thermal expansion mismatch between the target and backing plate. Titanium is expensive, however, and is thus not desirable as a material from which to fabricate backing plates. Less expensive materials, such as aluminum, have significantly different thermal expansion coefficients from that of tungsten-titanium though, resulting in target/backing plate assembly distortion and/or target cracking.
It is therefore a main objective of the present invention to provide a target/backing plate assembly and method of making same which can accommodate mismatch between target and backing plate coefficients of thermal expansion so as to not jeopardize the thermal and/or structural bond integrity between the target and backing plate.